Semiconductor devices such as logic and memory devices are typically fabricated by a sequence of processing steps applied to a substrate or wafer. Various features and multiple structural levels of the semiconductor devices are formed by these processing steps. For example, lithography among others is one semiconductor fabrication process that involves generating a pattern on a semiconductor wafer. Additional examples of semiconductor fabrication processes include, but are not limited to, chemical-mechanical polishing, etch, deposition, and ion implantation. Multiple semiconductor devices may be fabricated on a single semiconductor wafer and then separated into individual semiconductor devices.
Inspection processes are used at various steps during a semiconductor manufacturing process to detect defects on wafers to promote higher yield. As design rules and process windows continue to shrink in size, inspection systems are required to capture a wider range of physical defects on both unpatterned and patterned wafer surfaces while maintaining high throughput. Similarly, inspection systems are required to capture a wider range of physical defects on reticle surfaces.
One such inspection system is a scanning surface inspection system that illuminates and inspects a wafer surface. The wafer is scanned under an illumination spot until the desired portion of the wafer surface is inspected. Typically, a high-power, laser based illumination source generates illumination light with a non-uniform (e.g., Gaussian) beam intensity profile. However, it is generally desirable to project illumination light onto the specimen under inspection with an intensity distribution that is as uniform as possible over the field of view of the inspection system.
For example, in high-power, laser-based inspection systems, the power density of the incident laser beam is capable of damaging the wafer surface. For inspection systems employing a short-pulsed laser illumination source, substrate damage is primarily related to peak power density. An excessive amount of heat is generated by the interaction of the incident optical radiation with the wafer surface, particularly in areas of incidence subject to incident light with peak power density.
In another example, imaging systems generally rely on illumination light having an intensity distribution that is as uniform as possible over the field of view to effectively image the surface of the specimen.
One approach to generating a uniform intensity distribution from a non-uniform (e.g. Gaussian) beam source is to use only the center portion of the beam profile. While robust and simple, a significant amount of light is wasted; at significant system cost. In addition, care must be taken to properly dump the unused light while avoiding stray light issues.
Another approach involves the use of a diffractive optical element (DOE) that receives the non-uniform input beam and generates multiple secondary beams. By controlling the relative phase and position of the secondary beams, a DOE may generate a composite illumination light that approximates a uniform intensity distribution at the wafer surface.
Unfortunately, DOE elements are highly sensitive to periodic phase and intensity fluctuations in the illumination beam profile (wave front errors), and also to position of input beam with respect to DOE. Furthermore, a manufactured DOE is a fixed optical structure that typically cannot be adapted to accommodate changes in requirements for the final illumination profile. Similarly, a manufactured DOE cannot be actively altered to respond to changes in phase or intensity distribution of an input beam. In addition, DOEs are also relatively expensive both to design and manufacture compared to standard optical components, such as spherical lenses and flat mirrors.
Another approach to generating a uniform distribution involves the use of diffusers. However, diffusers share many of the same problems described hereinbefore with respect to DOEs. In addition, in applications involving coherent illumination, diffusers may give rise to undesired speckle.
Another approach to generating a uniform distribution involves the use aspheric optics. However, aspheric optics share many of the same problems described hereinbefore with respect to DOEs.
In some examples, multiple, independent light sources may be employed to generate a uniform distribution. However, the additional system cost is undesirable.
In general, the disadvantages of existing beam forming systems include low efficiency, sensitivity to aberrations, complexity, and poor flexibility. Typically, systems for generating a uniform distribution from a single Gaussian beam without light dumping are designed to break the input beam into multiple copies, and manipulate each copy individually. Such manipulations include attenuation, phase delay, or repositioning in space. However, such manipulations are significantly impacted by interference between the copies, particularly when real-world aberrations of the input beam are taken into account.
Hence, improvements to scanning inspection systems are desired to mitigate interferences among multiple illumination beams employed to illuminate a specimen under inspection.